FIG. 4 is a view used to describe a charge operation of a secondary battery by CCCV (Constant Current Constant Voltage) charge according to the background art. FIG. 4 shows a case where a nonaqueous electrolyte secondary battery, for example, a lithium-ion secondary battery, is charged and it shows a terminal voltage Vt and a charge current Ic of the lithium-ion secondary battery and the SOC (State of Charge) of the secondary battery.
In the CCCV charge, CC (Constant Current) charge is performed first and then it is switched to CV (Constant Voltage) charge when the terminal voltage Vt across a lithium-ion secondary battery has reached an end voltage Vf, after which a charge current Ic is kept reduced so as to maintain the terminal voltage Vt at the end voltage Vf. Full charge is determined when the charge current Ic has dropped to a current value Ia and feeding of a charge current is stopped (for example, see Patent Document 1).
In the CC charge, a charge current Ic, which is a predetermined constant current I2, is fed to a nonaqueous electrolyte secondary battery as a charge current. Accordingly, the positive electrode potential Pp of the nonaqueous electrolyte secondary battery rises whereas the negative electrode Pm drops with an increase of the SOC (State of Charge).
FIG. 5 is a view showing a case where a lithium-ion secondary battery is charged to describe a relation of the SOC, the positive electrode potential Ppo and the negative electrode potential Pmo when the circuit is open and the positive electrode potential Ppc and the negative electrode potential Pmc when the circuit is closed and a charge current is fed therein. As is shown in FIG. 5, the lithium-ion secondary battery is charged first by the CC charge and the positive electrode potential Ppo rises whereas the negative electrode Pmo drops with an increase of the SOC. In this instance, a voltage drop caused by the internal resistance of the battery occurs due to the charge current flowing into the lithium-ion secondary battery. Hence, the positive electrode potential Ppc and the negative electrode potential Pmc, which are the actual electrode potentials during the charge, become higher than the positive electrode potential Ppo and lower than the negative electrode potential Pmo, respectively.
When the negative electrode potential Pmc has dropped to 0 V because of the CC charge, a potential difference between the positive electrode potential Ppc and the negative electrode potential Pmc, that is, the terminal voltage across the lithium-ion secondary battery, is found to be a reference voltage Ve, for example, 4.2 V. In the case of a unit cell, the end voltage Vf at which the CC charge is ended is set to the reference voltage Ve. In the case of a set battery formed by connecting a plurality of cells in series, it is set to a value found by multiplying the reference voltage Ve by the cell number of the lithium-ion secondary batteries in series.
When the terminal voltage across the lithium-ion secondary battery has reached the end voltage Vf, that is, when the terminal voltage across each cell has reached the reference voltage Ve and the negative potential Pmc has dropped to substantially 0 V, the charger switches the CC charge to the CV charge.
Incidentally, a lithium-ion secondary battery has a characteristic that the battery characteristic, such as the cycle life, deteriorates when a charge current is large because the charge efficiency, which is a ratio of a current contributing to a charge reaction with respect to the charge current, decreases due to the internal resistance of the battery and the battery temperature rises as Joule heat generated by a charge current that does not contribute to the charge reaction consequently increases. In the charge reaction of the lithium-ion secondary battery, lithium in lithium cobalt oxide in the positive electrode turns into lithium ions that migrate between layers of negative electrode carbon. Hence, the lithium-ion secondary battery has a characteristic that the battery characteristic readily deteriorates also when the terminal voltage across the cell nears to the reference voltage Ve (for example, 4.2 V) because it becomes difficult for lithium ions to enter between layers of the negative electrode carbon and the charge efficiency deteriorates significantly, which increases Joule heat generated by a charge current that does not contribute to the charge reaction.
Under these circumstances, deterioration of the battery characteristic caused by the CC charge is conventionally suppressed by setting the reference current value Ie, which is a current at which the battery characteristic does not deteriorate substantially when the terminal voltage across the cell is the reference voltage Ve, to a charge current value I2 of the CC charge.
Meanwhile, the reference current value Ie tends to decrease recently as the capacity of the lithium-ion secondary battery increases. Accordingly, when the reference current value Ie is used as the charge current value I2 of the CC charge, there arises an inconvenience that a charge time is extended because of the synergistic effect of an increase of the capacity and a decrease of the charge current. To overcome this inconvenience, as is shown in FIG. 4, the charge current value I2 is increased to exceed the reference current value Ie to a maximum extent within a range to have a cycle life required for the lithium-ion secondary battery to meet market needs, so that the charge time is shortened while allowing the cycle life to become shorter to some degree.
However, there are market needs to shorten the charge time on one hand, there are market needs to prolong the cycle life on the other hand. It is therefore unfavorable to shorten the cycle life in order to shorten the charge time as described above. In addition, when the charge current value I2 of the CC charge is increased to a current value exceeding the reference current value Ie, the battery is charged in a overvoltage state where the charge voltage exceeds the end voltage Vf during a delay time until the CC charge is switched to the CV charge since it is detected that the terminal voltage Vt across the lithium-ion secondary battery has reached the end voltage Vf. When such a charge and discharge cycle is repeated, there is an inconvenience that deterioration of the battery characteristic caused by an overvoltage is accumulated and increased.
Patent Document 1: JP-A-6-78471